This invention relates generally to field emission display devices, and in particular, to methods of manufacturing cathodes for field emission devices.
Field emission displays (FEDs) are flat panel display devices that combine the size and portability advantages of liquid crystal displays (LCDs) with the performance of conventional cathode ray tubes (CRTs). FED devices typically include a field emission cathode positioned opposite a flat screen coated with phosphors. The phosphors emit light in response to bombardment by electrons from the cathode to produce an image. The field emission cathode emits electrons when subjected to an electric field of sufficient strength. The cathode typically includes thousands of microscopic emitter tips for each pixel of the screen. It is principally the emissive nature of the cathode that gives FEDs the thin, flat screen features of an LCD with the viewing angle, brightness, and response speed of a CRT.
While FEDs are potentially very attractive devices, there are two limiting factors in the widespread adoption of the technology. First, FED devices are difficult to manufacture, particularly the FED cathodes. The field emission cathodes typically comprise sharp-tip metal electron emitters having a tip radius on the order a few tens of nanometers, such as molybdenum emitter cones and graphite or diamond emitter cones. A variety of processes have been developed for producing FEDs, but they are complicated and expensive. However, copending application Ser. No. 09/373,028, filed on even date herewith, entitled xe2x80x9cField Emission Cathodes and Process for Their Manufacturexe2x80x9d, Benjamin E. Russ et al., incorporated by reference herein in its entirety, discloses an inexpensive and improved manufacturing process for field emission cathodes in which the emitting material is deposited by electrophoretic deposition.
The second limiting factor for FEDs is that FEDs are inherently unstable with respect to electron emission. Behaving similarly to a short circuit, when the electron emission begins, it draws all current to an initial emission site, preventing other areas adjacent to the initial emission site from emitting. It has been found that by placing a resistive material, on the order of 300,000 ohm-cm, between the conductive cathode substrate and the emitting material, the emission is more uniform and controlled.
This resistive material is typically vacuum sputtered onto the cathode material, followed by etching. The resistive material is sometimes patterned like the cathode structure and sometimes is left as a uniform coating over all the cathode material. Control of the thickness and uniformity of the resistive material coating is extremely important, but is difficult to achieve with the sputtering process. Moreover, this process can be very expensive using semiconductor manufacturing techniques.
Thus, despite the advances made in the manufacturing of field emission cathodes, there still remains a need for improving the electron emission stability of FEDs. More specifically, there remains a need for an inexpensive, improved technique for forming a resistive material layer where thickness and uniformity may be controlled.
The present invention provides an efficient and inexpensive method for depositing a resistive material directly onto the cathode portion of a substrate in a field emission display device, with very little waste of the resistive material and with precise control. To this end, and in accordance with the principles of the present invention, particles of a resistive material are deposited on a conducting layer overlying an insulating layer by electrophoretic deposition and an electron emitting material is deposited onto the resistive material layer to produce a cathode. Desired properties of a field emission cathode include requisite adhesive strength of the emitting particles to the resistive material layer and of the resistive material to the conducting layer, sufficient emission when an electric field is applied to the cathode, and spatial and temporal stability of the field emission. These properties may be achieved in the present invention by electrophoretic deposition, in which resistive material particles are suspended in a non-aqueous medium and deposited onto a conducting substrate under the influence of an electric field.
According to a further aspect of the present invention, by controlling the composition of the deposition bath, an electrophoretic deposition process can be used to efficiently and inexpensively produce field emission cathodes of the desired characteristics. The deposition bath for the field emission cathode advantageously includes an alcohol, a charging salt, water, and a dispersant. The dominant component of the deposition bath is a reasonably hydrophilic alcohol such as propanol, butanol, or an octanol. A charging salt such as Mg(NO3)2, La(NO3)3, or Y(NO3)3, at a concentration of between about 10xe2x88x925 to 10xe2x88x921 moles/liter, is added to the alcohol. The metal nitrates will partially dissociate in the alcohol and the positive dissociation product will adsorb onto the resistive material particles charging them positively. The water content will have a significant effect on the adhesion of particles to the conductive layer and to each other. The dissolved charging salt will react with water to form a hydroxide that will serve as a binder. Water content of between about 1% and about 30% by volume is used to increase the adhesion of deposited particles. The deposition bath also includes a dispersant, for example, glycerin, at a concentration of from 1% to 20% by volume of the deposition bath. Particularly advantageous results may be obtained for deposition of particles of a resistive material in the size range between about 0.01 and 0.5 xcexcm in a deposition bath of isopropyl alcohol containing 10xe2x88x923 molar Mg(NO3)2 with 3% water by volume and 1% glycerin by volume.
The field emission cathodes produced according to the method of the present invention are expected to exhibit emission with excellent spatial and temporal stability. The resistive layer may be uniformly deposited with good adhesion to the underlying substrate. The field emission cathodes so produced can be used as an electron source in a field emission display device.
These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof.